The present invention relates to an image forming apparatus, more particularly to an electrophotographic image forming apparatus such as printers, copying machines and facsimile machines.
It is known for this kind of an electrophotographic image forming apparatus that several kinds of chemical substances are emitted during imaging operation. Typical chemical substances to be emitted (chemical emission) include ozone generated during charging of a photoconductor and toner powder dust generated during developing or fixing operation. Conventional solutions to the chemical emission include taking measures against the emission source of such chemicals emission so as to decline the emission amount itself, and providing a filter to prevent emitted substances from being discharged to the outside of the apparatus. For example in JP H5-19582 A, when ozone is sucked, a duct through which the ozone passes is structured to have a smaller opening area, so that the ozone takes longer time to pass through the filter.
However, with a recent increase in awareness of global environmental conservation, fine particles which are substances different from ozone or toner powder dust, particularly ultra fine particles (with a particle size of 100 nm or less) generated from electrophotographic image forming apparatuses have come to be seen as a problem. Up to now, it has been unknown where in the inside of an image forming apparatus the ultra fine particles are generated, and therefore it has been impossible to take effective measures for the problem.
As a result of the investigation conducted by the inventor of the present invention, it was found out that in an electrophotographic image forming apparatus, the ultra fine particles are mainly generated in a fixing device. Further, if an exhaust fan, which is for sucking air of the fixing device containing the ultra fine particles, is constantly used at a maximum rotation frequency, the fixing device is cooled, which results in deteriorated fixability and increased energy consumption.
As shown in FIG. 10A, a general fixing member 300 is composed of layers including a base material 301 made of a cylindrical core metal or an annular endless belt, a rubber layer 302 provided so as to cover the outer surface of the base material 301, and an outer layer 303 provided so as to cover the outer surface of the rubber layer 302. In this example, a heater 305 (equivalent to a heater 133 in FIG. 1) is provided in an internal space of the base material 301 for heating the fixing member 300 to a specified target temperature (a fixing temperature in the range of 180° C. to 200° C.). The rubber layer 302, which is made of a silicone rubber material, has heat tolerance to the fixing temperature and elasticity for allowing for the width of a nip section. The outer layer 303 is made of, for example, PFAs (tetra fluoro ethylene-PerFluoro Alkylvinyl ether copolymers) for aiding release of a sheet (recording material such as paper sheets) which passed through the nip section. An end portion 302e of the rubber layer 302 and an end portion 303e of the outer layer 303 are positioned on the inside of an end portion 301e of the base material 301 respectively with respect to a direction along a central shaft C of the base material 301.
According to the investigation conducted by the inventor of the present invention, siloxane (designated by reference sign G) is generated in the form of ultra fine particles from the silicone rubber material which constitutes the rubber layer 302 when the base material 301, the rubber layer 302 and the like are heated with the heater 305 (reference sign H shows heat rays) as shown in FIG. 10B. Since the outer layer 303 made of PFAs and the like typically has a nature hard to transmit the ultra fine particles (gas barrier property), siloxane G is emitted from the end portion 302e of the rubber layer 302.
Examples of siloxanes include hexamethyldisiloxane (abbreviation: L2, molecular formula: C6H18O1Si2), hexamethylcyclotrisiloxane (abbreviation: D3, molecular formula: C6H18O3Si3) octamethyltrisiloxane (abbreviation: L3, molecular formula: C8H24O2Si3) octamethylcyclotetrasiloxane (abbreviation: D4, molecular formula: C8H24O4Si4), decamethyltetrasiloxane (abbreviation: L4, molecular formula: C10H30O3Si4) decamethylcyclopentasiloxane (abbreviation: D5, molecular formula: C10H30O5Si5) dodecamethylpentasiloxane (abbreviation: L5, molecular formula: C12H36O4Si5), and dodecamethylcyclohexasiloxane (abbreviation: D6, molecular formula: C12H36O6Si6).
An experiment conducted by the inventor of the present invention indicates that emission of siloxane G rapidly increases at the moment when the temperature of the fixing member 300 approximates 180° C. and the emission stops after the elapse of about 2 minutes. Such conditions for discharge of fine particles from the fixing member 300 (rubber layer 302 in particular) are called “initial burst conditions”.